Generally, the injector of a vehicle engine supplies liquid fuel to the interior of a combination chamber. This process is accompanied by an atomization process to improve ignition performance and the combustion efficiency of fuel.
The above-described atomization process of liquid fuel occurs as the pressure energy of fuel is converted into velocity energy while the liquid fuel is passed through the orifice of an injector. The velocity of the liquid fuel, which is passed through the orifice of the injector at the early stage of fuel injection, and the tip velocity of sprayed fuel, which is generated at the early stage of fuel injection, play important roles in the atomization of liquid fuel.
Particularly, in a diesel engine into which fuel is sprayed in high-pressure surroundings, the tip velocity of sprayed fuel greatly affects the wall impinging and wall wetting of fuel particles, and the degree of penetration of sprayed particles in a cylinder.
For these reasons, various methods are employed to measure the velocity of sprayed particles. Of the methods, the representative methods include a Phase Doppler Analysis (PDA) method that uses the scattering of light caused by particles, and Particle Image Velocimetry (PIV) and Particle Trajectory Velocimetry (PTV) methods that graphically process the tracks of sprayed particles.
However, in the above-described methods, it is impossible to measure the velocity of sprayed fuel as liquid fuel exists in a liquid column or liquid ligament form at the early stage of fuel injection. Furthermore, the PDA method enables real-time measurement, but uses a laser as a light source, so that equipment prices are very high.